The combination of geometrically variable compression (VCR) and early intake valve closure (EIVC) proved to offer high potential for increasing efficiency of gasoline engines. While early intake valve closure reduces pumping losses, it is detrimental to combustion quality and residual gas tolerance due to a loss of temperature and turbulence. Large geometric compression ratio at part load compensates for the negative temperature effect of EIVC with further improving efficiency. By optimizing the stroke/bore ratio, the reduction in valve cross section at part load can result in greater charge motion and therefore in turbulence. Turbocharging means the basis to enable an increase in stroke/bore ratio, called β in the following, because the drawbacks at full load resulting from smaller valves can be only compensated by additional boosting pressure level.
In this publication, the potential of an optimized stroke/bore ratio in combination with EIVC and VCR at part and full load is effectively assessed with a combination of 1D/QD and 3D-CFD simulation. In order to obtain reliable results, additional model approaches, such as turbulence, combustion and knock models were applied in 1D/QD simulation.
Increasing the stroke/bore (β) ratio from 1.07 to 1.4 is capable of increasing the turbulent kinetic energy by 30 % at the operating point n = 2000 rpm/BMEP = 2 bar with EIVC. Together with a compression ratio of 16.5, this results in 7 % higher residual gas tolerance which significantly reduces pumping losses. The reduced wall heat losses on increasing the stroke/bore ratio leading to an even further increased optimum compression ratio by 1.5 units to 18. By combining EIVC, high compression and optimum stroke/bore ratio of 1.4, it was possible to determine fuel consumption of BSFC = 317 g/kWh at 2000 rpm/2 bar (FMEP = 0.65 bar). This means a reduction in fuel consumption of about 9 % compared to the basis with stroke/bore ratio 1.07, EIVC and compression ratio 10.
By taking into consideration new driving cycles and new powertrain concepts, the low-load range is becoming increasingly less relevant or significant. Therefore, the useful maximum compression ratio for part load decreases. A reasonable spread of geometric compression ratio for part and full load would appear to be just 4 units in the range of 12 - 16.